Method and apparatus to generate and monitor optical signals and control power levels thereof in a planar lightwave circuit

ABSTRACT

An optical transmitter includes an external cavity laser array formed in a PLC, a trench-based detector array and an AWG. The external cavity laser is formed using an array of substantially similar laser gain blocks and an array of gratings formed in waveguides connected to the gain blocks. Each grating defines the output wavelength for its corresponding external cavity laser. Each detector of the detector array includes a coupler to cause a portion of a corresponding laser output signal of the laser array to propagate through a first sidewall of a trench and reflect off a second sidewall of the trench to a photodetector. In one embodiment, the photodetector outputs a signal indicative of the power level of the reflected signal, which a controller uses to control the laser array to equalize the power of the laser output signals.

FIELD OF THE INVENTION

[0001] Embodiments of the invention relate generally to opticalcommunication systems and more specifically but not exclusively tomulti-wavelength optical signal generators for use in opticalcommunication systems.

BACKGROUND INFORMATION

[0002] Optical signal generators (e.g., lasers) are widely used inoptical transmitters in wavelength division multiplexed (“WDM”) opticalcommunication systems. Some optical signal generators use a distributedfeed-back (DFB) laser for each channel of the WDM system. The opticalsignals generated by the multiple DFB lasers are then combined usingelements such as arrayed waveguide grating based multiplexer or anyother multiplexer. However, because a separate DFB laser is used foreach channel, the optical transmitters tend to have increased complexityand cost. Further, the output wavelength of a DFB laser is relativelysensitive to temperature changes (i.e., thermal wavelength drift). Forexample, applications using DFB lasers need to provide special attentionto wavelength stability over the desired temperature range, therebyincreasing complexity and cost. Thus, reduction of this temperaturedependency is important task on its own merits.

[0003] In addition, the optical transmitters typically require circuitryto monitor the power of the optical signal of each channel of the WDMsystem. This power monitoring circuitry is generally separate from theDFB laser devices (i.e., discrete), increasing the complexity and costsof fabricating the optical transmitters.

[0004] Still further, in many WDM applications, the power levels of theoptical signals (of the various WDM channels) are equalized. Someapproaches use separate attenuator circuits (e.g., thermo-optic MachZendher devices) to equalize the power between channels. Again, suchcircuitry tends to increase the complexity and cost of fabricatingoptical transmitters.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Non-limiting and non-exhaustive embodiments of the presentinvention are described with reference to the following figures, whereinlike reference numerals refer to like parts throughout the various viewsunless otherwise specified.

[0006]FIG. 1 is a block diagram illustrating an optical transmitterimplemented on a planar lightwave circuit (PLC), according to oneembodiment of the present invention.

[0007]FIG. 2 is a block diagram illustrating an external cavity laserarray for use in the optical transmitter of FIG. 1, according to oneembodiment of the present invention.

[0008]FIG. 3 is a block diagram illustrating an implementation of thetrench-based detector array of FIG. 1, according to one embodiment ofthe present invention.

[0009]FIG. 4 is a block diagram illustrating a photodetector andasymmetric trench implementing an optical detector of the detector arrayof FIG. 3, according to one embodiment of the present invention.

[0010]FIG. 5 is a diagram illustrating a cross section of a PLC showingan implementation of the photodetector and trench of FIG. 4, accordingto one embodiment of the present invention.

[0011]FIG. 5A is a diagram illustrating a cross section of a PLC showingan implementation of the photodetector and trench of FIG. 4 and thepower controller of FIG. 1, according to another embodiment of thepresent invention.

[0012]FIG. 6 is a flow diagram illustrating the operational flow of theoptical transmitter in controlling the optical power of each channel ofthe optical transmitter's output, according to one embodiment of thepresent invention.

DETAILED DESCRIPTION

[0013] In the following detailed description, numerous specific detailsare set forth. However, it is understood that embodiments of theinvention may be practiced without these specific details. In otherinstances, well-known circuits, structures, and techniques have not beenshown in detail in order to not obscure the understanding of thisdescription. These embodiments are described in sufficient detail toenable those of ordinary skill in the art to practice the invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the invention is defined only by theappended claims.

[0014]FIG. 1 illustrates an optical transmitter 100 implemented on aplanar lightwave circuit (PLC) 101, according to one embodiment of thepresent invention. In some embodiments, PLC 101 can have circuitry inaddition to optical transmitter 100.

[0015] In this embodiment, optical transmitter 100 includes a laserarray 110, a trench-based detector array 120, an arrayed waveguidegrating (AWG) 130, a grating array 140, and a power controller 150. Insome embodiments, power controller 150 is not integrated on PLC 101, asindicated by dashed lines in FIG. 1.

[0016] This embodiment of optical transmitter 100 is interconnected asfollows. Laser array 110 has N optical signal output waveguides 160₁-160 _(N) each having a corresponding grating of grating array 140formed in these output waveguides. Thus, in this embodiment, gratingarray 140 has N gratings. As will be described below in more detail,each grating of grating array 140 defines a wavelength for a channel ofan N-channel WDM system.

[0017] Detector array 120 has N optical input ports connected tocorresponding waveguides of waveguides 160 ₁-160 _(N). Detector array120 also has N output ports, each connected to a corresponding waveguideof waveguides 170 ₁-170 _(N). In this embodiment, detector array alsohas an electrical output port that is connected to an electrical inputport of power controller 150 via a line 191. Power controller 150 alsohas an electrical output port, which is connected to an electrical inputport of laser array 110 via a line 192.

[0018] AWG 130 has N input waveguides connected to a correspondingwaveguide of waveguides 170 ₁-170 _(N). AWG 130 is configured to combineall of the optical signals received at its input ports and output themvia a waveguide 180.

[0019] In an alternative embodiment, laser array 110 may have a secondset of output waveguides (e.g., on the opposite side of the laser gainblocks to propagate light passing through the laser gain blockreflectors). Detector array 120 may be connected to these outputwaveguides rather than in the “primary” output ports of laser array 110.

[0020] In operation, laser array 110 and grating array 140 form an arrayof external cavity lasers providing optical output signals with Ndifferent wavelengths (i.e., a wavelength for each channel of anN-channel WDM system). These external cavity lasers are described belowin more detail in conjunction with FIG. 2.

[0021] Detector array 120, in this embodiment, is used to monitor thepower of each of the N optical signals generated by the external cavitylasers formed by laser array 110 and grating array 140. Detector array120 is described below in more detail in conjunction with FIGS. 3 and 4.In some embodiments, different power monitoring circuitry can be usedwith the external cavity lasers formed by laser array 110 and gratingarray 140. Still further, in some embodiments, different optical signalsources can be used with detector array 120.

[0022] Detector array 120 provides signals to power controller 150indicative of the power of each the N optical signals. Theinterconnection between detector array 120 and power controller 150 isdescribed below in conjunction with FIGS. 5 and 5A.

[0023] Power controller 150 can then provide control signals to laserarray 110 to adjust the power of each of the N optical signals asdesired for the application. One embodiment of this operation isdescribed below in conjunction with FIG. 6. In addition, powercontroller 150 can include a laser drive circuit (or other circuitry) todirectly modulate the optical signals output by laser array 110 (e.g.,on-off keying (OOK)). The N optical signals are then multiplexed and/orcombined by AWG 130.

[0024] These embodiments of optical transmitter 100 can provide severaladvantages. For example, in embodiments that use the external cavitylasers formed by laser array 110 and grating array 140, the externalcavity lasers can be implemented using substantially identical lasergain blocks (with the wavelengths being defined by grating array 140),rather than multiple lasers of different wavelengths. Further, the lasergain blocks can be fabricated as a “laser bar” (i.e., multiple blockscleaved as a “bar” from a wafer) rather than being singulated. Anotheradvantage of using external cavity lasers with laser bar is thatintegration and positioning/alignment of the laser bar becomes a lessstringent process compared to that of separate laser gain modules (inwhich each module is aligned with its corresponding waveguide with aspecified accuracy. Other advantages of the external cavity lasers aredescribed below in conjunction with FIG. 2.

[0025] Another advantage, in embodiments that use detector array 120, isthat detector array 120 can be easily integrated into PLC 101 due to itstrench-based design. Thus, the use of detector array 120 canadvantageously reduce the size and costs of fabricating opticaltransmitter 100.

[0026] In addition, in embodiments using detector array 120, thefeedback control of the laser source (i.e., laser array 110) viadetector array 120 and controller 150 can eliminate the need forseparate variable attenuators that are typically located downstream ofthe laser source to equalize power between channels. This approach cansimplify the design and can reduce size and costs. This approach isdifferent from some current systems based on DFB or FP (Fabry-Perot)lasers in which feedback is used only to stabilize the wavelength powerwithin relatively small range. However, some embodiments using detectorarray 120 can provide wavelength stabilization as well as powerequalization.

[0027]FIG. 2 illustrates an external cavity laser array 200 implementedusing laser array 110 (FIG. 1) and grating array 140 (FIG. 1), accordingto one embodiment of the present invention. Detector array 120 (FIG. 1)and power controller 150 (FIG. 1) are omitted from FIG. 2 to promoteclarity.

[0028] This embodiment of external cavity laser array 200 includes lasergain blocks 210 ₁-210 _(N), which are part of laser array 110. In thisembodiment, laser gain blocks 210 ₁-210 _(N) are substantiallyidentical, and each has a reflective surface serving as one mirror of anexternal cavity. In one embodiment, laser gain blocks 210 ₁-210 _(N) areimplemented using the laser diodes of an array of edge emitting laserdevices. In other embodiments, laser gain blocks 210 ₁-210 _(N) areimplemented using an array of vertical cavity surface emitting lasers(VCSELs).

[0029] This embodiment of external cavity laser array 200 also includesgratings 220 ₁-220 _(N), which are part of grating array 140. Gratings220 ₁-220 _(N) serve as the other mirrors for the array of externalcavity lasers. In this embodiment, gratings 220 ₁-220 _(N) have areflectivity of about 60%, although in other embodiments a reflectivityranging from about 50% to 70% may be used. Each of gratings 220 ₁-220_(N) are designed for the corresponding block of laser gain blocks 210₁-210 _(N) according to the wavelength defined by the grating and themodes of the external cavity formed thereby.

[0030] In one embodiment, gratings 220 ₁-220 _(N) are implemented assilicon-based waveguide Bragg gratings (WBGs) formed in the waveguidesimplementing the input ports of AWG 130 (which also serve as the outputports of laser array 110). For example, the WBGs may have alternatingregions of doped and undoped (or doped with different dopants) siliconresulting in alternating regions of different indices of refraction.These regions would be made with the proper width and spacing for thedesired Bragg wavelength. Alternatively, the WBGs may have alternatingregions of different materials (e.g., silicon and oxide, or silicon andpolysilicon). By using a silicon-based waveguide grating to define theoutput wavelength of each channel, wavelength drift under temperaturechanges can be reduced (e.g., DFB lasers typically have a drift of about0.1 nm/° C. compared to about 0.01 nm/° C. for a silicon-based WBG).

[0031] In yet another embodiment, the WBGs may be tunable (e.g., thermaltuning) using currently available WBG technology so that a fine-tuningof the output wavelength can be performed. Such an embodiment can beadvantageously used in WDM applications requiring precise wavelengthallocation.

[0032] The spacing between the ports of the AWG 130 and the lengths ofwaveguides in the grating region of AWG 130 are configured to multiplexthe output wavelength of each external cavity laser (defined by theBragg wavelengths of the WBGs) to the output port connected to waveguide180. Thus, AWG 130 would not direct other wavelengths that might bepresent in the optical signals output by laser array 110 to the outputport connected to waveguide 180.

[0033] One advantage of this approach is that laser gain blocks 210₁-210 _(N), can be fabricated as a single “bar” of substantiallyidentical active components to assemble 1×N wavelength system, operatingin conjunction with grating array 140 to output the desired wavelengthsof the WDM system. The bar is a piece of laser material wafer that isdiced so that it incorporates the multiple substantially identical lasergain blocks, which can reduce fabrication costs compared to singulatedlaser gain blocks and, in addition, simplify attachment and alignment ofthe laser gain blocks with waveguides 160 ₁-160 _(N).

[0034]FIG. 3 schematically illustrates an implementation of thetrench-based detector array 120 (FIG. 1), according to one embodiment ofthe present invention. In this embodiment, each detector of detectorarray 120 includes a tap coupler, a tap waveguide and a photodetector.More particularly, as shown in FIG. 3, tap couplers 300 ₁-300 _(N) arerespectively connected to waveguides 160 ₁-160 _(N). Tap couplers 300₁-300 _(N) have tap output ports respectively connected to tapwaveguides 310 ₁-310 _(N), which in turn are respectively connected tophotodetectors 320 ₁-320 _(N). In addition, tap couplers 300 ₁-300 _(N)have “main” output ports respectively connected to waveguides 170 ₁-170_(N). In one embodiment, tap couplers 310 ₁-310 _(N) are each configuredto tap about 5% of the power of a received optical signal to itscorresponding photodetector. For example, the tap couplers can beimplemented using splitters or evanescent couplers configured to providethe desired power allocation between output ports. Further, in thisembodiment, photodetectors 320 ₁-320 _(N) are disposed on the surface ofPLC 101 (FIG. 1) as described below in conjunction with FIG. 4.

[0035]FIG. 4 illustrates in cross section an arrangement of an opticaldetector of detector array 120 (FIG. 3), according to one embodiment ofthe present invention. The other N−1 optical detectors are substantiallysimilar to the one depicted in FIG. 4. In this embodiment, an asymmetrictrench 400 is formed in PLC 101 substantially perpendicular to thedirection of light propagation in tap waveguide 310 ₁. In particular,one sidewall 400 ₁ of trench 400 is angled of about 98 to 105 degreesrelative to the direction of propagation of the optical signal. In thisembodiment, the other sidewall of trench 400 is angled at about 45degrees relative to the direction of propagation of the optical signal,spaced about 10 to 50 microns away from sidewall 400 ₁. In addition, areflective surface 410 is formed on this sidewall. In one embodiment,reflective surface 410 is formed with a layer of metal such as Al, Au,TiW or combinations of different layers. Other embodiments use mirrorsimplemented using alternating layers of materials having differentrefractive indices (e.g., alternating layers of dielectric andsemiconductor materials).

[0036] In this embodiment, photodetector 320 ₁ is disposed above trench400. In one embodiment, photodetector 3201 (as well as photodetectors320 ₂-320 _(N)) are formed on a die that is attached to PLC 101 (FIG. 1)so that the photodetectors are aligned with trench 400. In someembodiments, a photopolymer is used to attach the die to PLC 101.

[0037] In operation, tap coupler 300 ₁ (FIG. 3) taps a portion of theoptical signal received via waveguide 160 ₁. In one embodiment, theoptical signal propagated via waveguide 160 ₁ is generated by laser gainblock 210 ₁ (FIG. 2). The tapped portion of the optical signal ispropagated through tap waveguide 310 ₁ (indicated by arrow 420). Thisoptical signal exits through sidewall 400 ₁ of trench 400 and isreflected towards photodetector 320 ₁ by reflective surface 410.Photodetector 320 ₁ converts the received optical signal into anelectrical signal, which is then provided to power controller 150 (FIG.1). For example, the die containing photodetectors 320 ₁-320 _(N) canuse flip-chip bonding, wire bonding, tape automated bonding (TAB) orother interconnect techniques to conduct the electrical output signalsfrom photodetectors 320 ₁-320 _(N) to power controller 150.

[0038]FIG. 5 illustrates a cross section of PLC 101 (FIG. 1) showing animplementation of photodetector 320 ₁, and trench 400 (FIG. 4),according to one embodiment of the present invention. In thisembodiment, photodetector 320 ₁ has a photodiode 500 formed on the sideof the die facing trench 400. A wire bond 510 is used to conduct theoutput signal generated by photodiode 500 to a conductive contact region515 formed on the surface of PLC 101. Alternatively, wire bond 510 canbe omitted and flip-chip bonding techniques can be used to interconnectthe die containing photodetector 320 ₁ to PLC 101. In this embodiment, acap 520 is attached to PLC 101 to hermetically (or semi-hermetically inother embodiments) seal the photodetectors and prevent extraneous lightfrom adding noise to the photodetector output signal and to protect thechips from degradation from environmental conditions (e.g., humidity).

[0039] In this embodiment, power controller 150 (FIG. 1) is external toPLC 101 (e.g., a processor or microcontroller mounted on a motherboard)and is connected to receive multiple signals from and provide power toPLC 101 via an electrical connector 530. In particular, connector 530 isconnected to receive electrical signals via trances on PLC 101 (notshown) via wire bonds such as wire bond 540 used to conduct the outputsignal of photodiode 500 to connector 530 (via contact region 515).Power controller 150 can then receive the output signal of photodiode500 via a pin 550 of connector 530.

[0040] In operation, optical signal 420 (from tap coupler 300 ₁ (FIG.3)) is propagated through tap waveguide 310 ₁ and reflected offreflective surface 410 of asymmetric trench 400 (FIG. 4). The reflectedoptical signal is received directly by photodetector 500, facing theasymmetric trench. In one embodiment, photodetector 500 converts opticalsignal 420 into an electrical signal having a current as a function ofthe power of optical signal 420. This electrical signal is provided topower controller 150 (FIG. 1) via wire bond 510, contact region 515,conductive trace (not shown) formed on the surface of PLC 101, wire bond540 and pin 550 of electrical connector 530. Power controller 150 thencontrols laser gain block 210 ₁ (FIG. 2) to generate its output opticalsignal at a desired power level. The operation of power controller 150is described below in conjunction with FIG. 6 in controlling the opticalpower level of each channel.

[0041] In an alternative embodiment, laser array 110 (FIG. 1) mayinclude a laser drive circuit (not shown) or other means to directlymodulate the output signals from laser array 110. Power controller 150can then be used to modulate the optical output signals as well as toequalize channel power. In addition, in some embodiments, powercontroller 150 can be configured to disable the laser gain block(s) ofunused channels.

[0042]FIG. 5A illustrates an alternative implementation of powercontroller 150 (FIG. 1) integrated on PLC 101 and photodetector 320 ₁and trench 400 (FIG. 4). In this alternative embodiment, photodiode 500Ais formed on the side of the die facing away from trench 400. Wire bond510 is used to conduct the output signal generated by photodiode 500A toan input lead of power controller 150.

[0043] In this embodiment, optical signal 420 is propagated through tapwaveguide 310 ₁, reflected off reflective surface 410 of the asymmetrictrench, through the back side of the die to photodetector 500A. Powercontroller 150 receives the electrical output signal of photodetector500A via wire bond 510, which in turn controls laser gain block 210 ₁ aspreviously described.

[0044]FIG. 6 illustrates the operational flow of optical transmitter 100(FIG. 1) in controlling the optical power of each channel of the opticaltransmitter's output, according to one embodiment of the presentinvention. Referring to FIGS. 1 and 6, power controller 150 operates asfollows in controlling the optical power level of each channel.

[0045] As represented by a block 601, multiple optical signals areprovided to PLC 101. In one embodiment, these optical signals aregenerated by laser array 110, which is integrated on PLC 101. In oneembodiment, laser array 110 can be tuned to independently adjust thepower level of each optical output signal outputted to waveguides 160₁-160 _(N).

[0046] The multiple optical signals, as represented by a block 603, arepropagated in waveguides formed in PLC 101. A relatively small portionof each optical signal is then tapped from the waveguides. In thisembodiment, detector array 120 taps off the small portions of eachoptical signal using tap couplers 300 ₁-300 _(N) (FIG. 3). In oneembodiment, detector array 120 taps about 5% of the power from eachoptical signal. Ideally, in that embodiment, each tapped portion ismatched with regard to percentage of power being tapped. This operationis represented by a block 605.

[0047] Then as represented by a block 607, the tapped portions of theoptical signals are converted into corresponding electrical signals. Inone embodiment, detector array 120 converts the tapped portions intoelectrical signals using photodetectors 310 ₁-310 _(N) (FIG. 3). In someembodiments, each electrical signal has a current level that isproportional to the power level of its corresponding tapped portion.

[0048] The power level of each optical signal outputted by opticaltransmitter 100 is controlled to a desired level, as represented by ablock 609. In one embodiment, power controller 150 controls the powerlevels of each channel to be substantially identical. More particularly,power controller 150 receives the electrical signals corresponding toeach channel from detector array 120, and responsive to the currentlevel of each signal, controls the corresponding laser gain block toappropriately decrease/increase its output power to achieve the desiredpower level. This process implements a control loop, as indicated by thereturn of the operational flow from block 609 back to block 601.

[0049] In some embodiments, power controller 150 is implemented usinganalog circuitry. In alternative embodiments, power controller 150 isimplemented using a microcontroller or other processor. Thesealternative embodiments can be advantageously used with laser arraysthat include a laser drive circuit so that power controller 150 can beprogrammed to modulate the laser output signals as well as equalize thepower.

[0050] Embodiments of method and apparatus to generate and monitoroptical signals and control power levels thereof in a PLC are describedherein. In the above description, numerous specific details are setforth (such as laser devices, AWGs, photodetectors, etc.) to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that embodiments of theinvention can be practiced without one or more of the specific details,or with other methods, components, materials, etc. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring the description.

[0051] Reference throughout this specification to “one embodiment” or“an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

[0052] In addition, embodiments of the present description may beimplemented not only within a semiconductor chip but also withinmachine-readable media. For example, the designs described above may bestored upon and/or embedded within machine readable media associatedwith a design tool used for designing semiconductor devices. Examplesinclude a netlist formatted in the VHSIC Hardware Description Language(VHDL) language, Verilog language or SPICE language. Some netlistexamples include: a behavioral level netlist, a register transfer level(RTL) netlist, a gate level netlist and a transistor level netlist.Machine-readable media also include media having layout information suchas a GDS-II file. Furthermore, netlist files or other machine-readablemedia for semiconductor chip design may be used in a simulationenvironment to perform the methods of the teachings described above.

[0053] Thus, embodiments of this invention may be used as or to supporta software program executed upon some form of processing core (such asthe CPU of a computer) or otherwise implemented or realized upon orwithin a machine-readable medium. A machine-readable medium includes anymechanism for storing or transmitting information in a form readable bya machine (e.g., a computer). For example, a machine-readable medium caninclude such as a read only memory (ROM); a random access memory (RAM);a magnetic disk storage media; an optical storage media; and a flashmemory device, etc. In addition, a machine-readable medium can includepropagated signals such as electrical, optical, acoustical or other formof propagated signals (e.g., carrier waves, infrared signals, digitalsignals, etc.).

[0054] The above description of illustrated embodiments of theinvention, including what is described in the Abstract, is not intendedto be exhaustive or to be limitation to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible, as those skilled in the relevant art willrecognize.

[0055] These modifications can be made to embodiments of the inventionin light of the above detailed description. The terms used in thefollowing claims should not be construed to limit the invention to thespecific embodiments disclosed in the specification and the claims.Rather, the scope is to be determined entirely by the following claims,which are to be construed in accordance with established doctrines ofclaim interpretation.

What is claimed is:
 1. An external cavity laser apparatus, comprising: aplurality of waveguides each having a first portion and a second portionformed in a semiconductor material; a plurality of laser gain blocks toprovide optical signals to the first portions of the plurality ofwaveguides, each laser gain block being substantially similar; aplurality of gratings formed in the first portions of the plurality ofwaveguides, the plurality of gratings defining a plurality ofwavelengths, wherein each waveguide of the plurality of waveguidespropagates in its second portion an optical signal defined by a gratingformed in its first portion; and an array waveguide grating (AWG) formedin the semiconductor material to receive a plurality of optical signalsfrom the second portions of the plurality of waveguides.
 2. Theapparatus of claim 1, wherein the first portions of the plurality ofwaveguides has a corresponding single grating of the plurality gratings,each grating of the plurality of gratings defining a differentwavelength.
 3. The apparatus of claim 1, wherein the AWG further tomultiplex the plurality of the optical signals received from the secondportions to a single output waveguide.
 4. The apparatus of claim 1,wherein the plurality of laser gain blocks are integrated on a singlemonolithic substrate.
 5. The apparatus of claim 1, wherein the pluralityof gratings are tunable.
 6. The apparatus of claim 1, wherein a gratingof the plurality of gratings has a reflectivity ranging from about 50%to 70% for a selected wavelength.
 7. A method for generating amulti-wavelength optical signal, the method comprising: generating aplurality of optical signals using a plurality of substantially similarlaser gain blocks; propagating the plurality of optical signals in aplurality of waveguides formed in a planar waveguide circuit (PLC); ineach waveguide of the plurality of waveguides, partially reflecting aselected wavelength using a grating formed in the waveguide, the gratingof each waveguide defining a selected output wavelength; andmultiplexing optical signals passing through the plurality of gratingsusing an arrayed waveguide grating (AWG) so that selected wavelengthsdefined by the plurality of gratings are multiplexed to a single outputwaveguide of the AWG.
 8. The method of claim 7 wherein generating theplurality of optical signals comprises generating the plurality ofoptical signal from laser gain blocks integrated on a single monolithicsubstrate.
 9. The method of claim 7 further comprising tuning a gratingof the plurality of gratings to reflect a selected wavelength.
 10. Themethod of claim 7, wherein partially reflecting comprises reflectingabout 50% to 70% of a received optical signal's power at the selectedwavelength.
 11. The method of claim 7, wherein the selected wavelengthof each grating is different.
 12. An optical detection apparatus,comprising: a waveguide formed in a semiconductor substrate to propagatean optical signal received at a first end of the waveguide; a trenchformed in the waveguide having a first sidewall and a second sidewall,the first and second sidewalls forming first and second angles with thewaveguide's propagation direction; a reflective surface formed on thesecond sidewall, and a photodetector to receive an optical signalpropagated in the waveguide, through the first sidewall and reflectedfrom the reflective surface on the second sidewall.
 13. The apparatus ofclaim 12, wherein the photodetector is positioned above the trench. 14.The apparatus of claim 12, wherein the first angle ranges from about 98to 105 degrees and the second angle ranges from about 43 to 47 degrees.15. The apparatus of claim 12, wherein the photodetector is attached tothe semiconductor substrate using a photopolymer.
 16. The apparatus ofclaim 12, further comprising a cap attached to the semiconductorsubstrate to enclose the trench and photodetector.
 17. The apparatus ofclaim 12, further comprising a coupler attached to the first end of thewaveguide, the coupler to receive an input optical signal and cause aportion of the input optical signal to propagate in the waveguide.
 18. Amethod, comprising: propagating an optical signal through a waveguideincluded in a semiconductor substrate, the waveguide having formedtherein a trench having a first sidewall and a second sidewall, thefirst and second sidewalls forming first and second angles with thewaveguide's propagation direction; and receiving the optical signalusing a photodetector, the optical signal being propagated through thefirst sidewall and reflected from the second sidewall to thephotodetector.
 19. The method of claim 18, further comprisingpositioning the photodetector above the trench.
 20. The method of claim18, wherein the first angle ranges from about 98 to 105 degrees and thesecond angle ranges from about 43 to 47 degrees.
 21. The method of claim18, further comprising propagating an input optical signal to a coupler,the coupler causing a portion of the input optical signal to propagatein the waveguide.
 22. An optical power control apparatus, comprising: anoptical signal source to output an optical signal, a portion of which ispropagated in a waveguide formed in a semiconductor substrate; a trenchformed in the waveguide having a first sidewall and a second sidewall,the first and second sidewalls forming first and second angles with thewaveguide's propagation direction, the second sidewall having areflective layer, the portion of the optical signal propagated in thewaveguide being propagated through the first sidewall and reflected fromthe second sidewall; a photodetector to output a signal indicative of apower level of the portion of the optical signal reflected from thesecond sidewall; and a controller to provide, to the optical signalsource, a control signal responsive to the signal outputted by thephotodetector, the optical signal source to control a power level of theoptical signal source responsive to the control signal.
 23. Theapparatus of claim 22, wherein the photodetector is positioned above thetrench.
 24. The apparatus of claim 22, wherein the first angle rangesfrom about 98 to 105 degrees and the second angle ranges from about 43to 47 degrees.
 25. The apparatus of claim 22, further comprising a capattached to the semiconductor substrate to enclose the trench andphotodetector.
 26. The apparatus of claim 22, further comprising acoupler to receive the optical signal and cause the portion of theoptical signal to propagate in the waveguide.
 27. A method, comprising:generating an optical signal; propagating a portion of the opticalsignal through a waveguide included in a semiconductor substrate, thewaveguide having formed therein a trench having a first sidewall and asecond sidewall, the first and second sidewalls forming first and secondangles with the waveguide's propagation direction; generating an outputsignal responsive to a power level of the portion of the optical signalusing a photodetector, the portion of the optical signal beingpropagated through the first sidewall and reflected from the secondsidewall to the photodetector; and controlling a power level of theoptical signal responsive to the output signal from the photodetector.28. The method of claim 27, further comprising positioning thephotodetector above the trench.
 29. The method of claim 27, wherein thefirst angle ranges from about 98 to 105 degrees and the second angleranges from about 43 to 47 degrees.
 30. The method of claim 27, furthercomprising propagating the optical signal to a coupler, the couplercausing the portion of the optical signal to propagate in the waveguide.